This invention relates to transducers for ultrasonic diagnostic imaging systems and, in particular, to two-dimensional ultrasonic transducer arrays.
Transducer arrays are presently in widespread use in ultrasonic diagnostic imaging. Compared to single element (single piston) transducers, array transducers permit the beam transmitted and received by the elements of the array to be electronically steered and focused. Beamformers which perform steering and focusing: of both transmit and receive beams of transducer arrays are commonly available.
The transducer array which is most prevalent is one comprising a single row of transducer elements. Such transducer arrays are known as one-dimensional or 1D arrays, and are operable as linear, curved linear, and phased array transducers. The 1D array transducer is so named because it comprises a single line or row of transducer elements and is able to steer and focus the beam in only one dimension, the image plane which is aligned with the longitudinal dimension of the array row. The beam can be steered over a wide range of directions in this image plane. Such transducers are well suited for scanning an image plane for the two-dimensional imaging of a plane or xe2x80x9cslicexe2x80x9d of the body.
Transducer arrays may also be formed of multiple rows of transducer elements, one form of which is the 1.5D transducer array. In a 1.5D transducer array, additional rows of transducer elements are located symmetrically on either side of a central row of elements or about the longitudinal center of the array. Rows of elements which are symmetrically located on either side of the longitudinal center are operated together, enabling the transducer to be electronically focused in the elevation dimension orthogonal to the longitudinal dimension. This means that the 1D transducer array can produce a two dimensional image which is xe2x80x9cthinxe2x80x9d in the elevational (slice thickness) dimension.
When a transducer array is formed of multiple elements in two dimensions without the restriction of symmetrical operation in the elevation dimension, the ultrasonic beams can be both electronically steered and focused over 360xc2x0 of azimuth and 180xc2x0 of inclination. This enables the transducer array to scan beams over a three dimensional volume, thereby providing fully electronic scanning for three dimensional (3D) ultrasonic imaging. Fully electronic scanning is desirable both for reliability and to obtain the beam scanning necessary for real time 3D imaging.
When a 2D array transducer scans in any direction in a 3D volume, it is desirable that certain criteria which provide for high image quality be met for all beam scanning orientations. For instance the antenna pattern of the beam should prevent deleterious grating lobes, which can contribute clutter to the received ultrasound signals. A desirable criterion for grating lobes in a 2D array is that the pitch of the array, the maximum center-to-center spacing of nearest neighbor rows of transducer elements in any direction, be no greater than approximately one-half wavelength (xcex/2), where xcex is generally taken to be the wavelength of a reference or center frequency of the transducer. An array with a pitch in excess of this criterion can contribute a relatively high degree of unwanted clutter to the desired image information. For a 1D array the pitch is the distance from one element to the next, but for 2D arrays adjacent elements extend in two dimensions which must be considered.
A 2D array for 3D imaging should also be capable of being manufactured in significant quantities at relatively low cost. If the 2D array can only be manufactured by exotic and expensive processes, its cost will be excessive. A 2D array of the desired performance criteria which can be manufactured using standard 1D transducer array processes is highly desirable.
In accordance with the principles of the present invention, a two dimensional ultrasonic transducer array is formed of a plurality of transducer elements which are closely packed in a hexagonal grid pattern. The close packing in the hexagonal grid affords an optimally small pitch for good grating lobe performance. In one embodiment rectilinear transducer elements are arranged in staggered rows to form the hexagonal pattern, which allows the array to be manufactured using conventional fabrication techniques. In another embodiment the array elements are composite elements, affording further ease in manufacturing. Preferably the composite elements are operated in the k31 mode, which affords a further ease in making electrical connections to the array elements.